Polyimide optical waveguide and method of manufacturing the same

ABSTRACT

A polyimide optical waveguide comprising a core made of polyimide whose refractive index is controlled to a predetermined value by electron beam irradiation, and a cladding set in contact with the core and having a refractive index lower than that of the core.

This is a division of application Ser. No. 08/213,918 filed Mar. 15,1994, abandoned in favor of Ser. No. 08/425,343, filed Apr. 19, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polyimide optical waveguide, and moreparticularly to a polyimide optical waveguide in which the polyimide,the refractive index of which is controlled by an electron beamirradiation to a desired value, is used as a core, and also to a facilefabrication of the polyimide optical waveguide.

2. Description of the Related Art

As an optical communication system is put to practical, thanks to thedevelopment of a low loss optical fiber, it is desired that variouskinds of devices for the optical communication be developed. Further,there are demands for an optical wiring technique, more particularly anoptical waveguide technique by which the optical devices are packaged inhigh density.

Generally, some conditions are required of an optical waveguide, forexample, low optical losses, facile fabrication, controllablecore-cladding refractive index ratio, and a high heat resistance.

As material of the optical waveguide having low optical losses, asilica-based material can be cited. As already proved in optical fiber,silica which has a good optical transmittance achieves an optical lossof 0.1 dB/cm or less at wavelength of 1.3 μm, if used in an opticalwaveguide. Manufacture of quartz glass optical waveguides, however,presents a number of problems including a manufacturing processeslengthy in terms of time, the high temperatures needed duringfabrication, and the difficulty of making optical waveguides with alarge area.

To solve these problems, attempts have been made to produce opticalwaveguides using plastics such as polymethylmethacrylate (PMMA), whichcan be manufactured at low temperatures and low cost. Conventionalplastic optical waveguides, however, have low resistance to hightemperature. Thus, there is a demand for plastic optical waveguidehaving an excellent heat resistance.

Among the various organic polymers currently available, polyimidesprovide very high resistance to heat. Hence, these materials have beenwidely employed in the field of electronics, to form an insulating filmbetween layers in the multilayer wiring or to form a flexible printboard. However, there has been no example in which polyimide is appliedfor an optical device such as an optical waveguide.

In view of this, the inventors of the present invention have studied anddeveloped a polyimide optical material which is applicable for anoptical waveguide. When polyimide is used in optical communicationapplications as the optical material, there are two important points.First, its transparency in the visible and near infrared regions isexcellent. Second, its refractive index can be controlled freely. Theinventors disclose a fluorinated polyimide with excellent transparencyin the visible and near infrared regions, in Jpn. Pat. Appln. KOKAIPublication No. 3-72528. Further, in Jpn. Pat. Appln. KOKAI PublicationNo. 4-8734, it is disclosed that the core-cladding refractive indexratio, as is needed for, for example, forming an optical waveguide, isquite controllable by copolymerizing such fluorinated polyimide.Furthermore, optical waveguides using fluorinated polyimide aredisclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 4-9807, 4-235505and 4-235506. The control of the refractive index difference between thecore layer for passing the light and the cladding layer for shutting thelight is achieved by adjusting the fluorine content in the polyimide.Namely, two kinds of fluorinated polyimide having the differentrefractive indices for the core layer and for the cladding layer areused respectively. Therefore, there may be some problems with a kind ofan optical waveguide; that is, the core layer and the cladding layer maybe different in thermal properties or the birefringence.

In the conventional method of manufacturing a polyimide opticalwaveguide, a reactive ion etching (RIE) method, which is used in asemiconductor manufacturing process, is generally employed. The RIEmethod is disadvantageous in that it comprises many steps. Therefore, afacile fabrication of polyimide optical waveguides, which comprises asmaller number of steps and by which the above-mentioned method may bereplaced, has been desired.

The inventors of the present invention have studied to solve theabove-mentioned problems and have found it possible to change therefractive index of polyimide films by irradiating the films with anelectron beam, as they disclose in Japanese Patent Application No.4-226549.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an optical waveguideand an improved method of manufacturing a polyimide optical waveguide inwhich the problems caused by the difference in thermal properties andbirefringence between the core and the cladding can be solved, withsimple steps.

In order to achieve the above-mentioned object, the polyimide opticalwaveguide according to the present invention comprises a core made ofpolyimide whose refractive index is controlled to a predetermined valueby electron beam irradiation; and a cladding in contact with the coreand having a refractive index lower than that of the core.

According to another aspect of the present invention, there is provideda polyimide optical waveguide comprising: at least one core made ofpolyimide whose refractive index is controlled to a predetermined valueby electron beam irradiation and at least one cladding having arefractive index controlled by electron beam irradiation and being lowerthan that of the at least one core, wherein the at least one core andthe at least one cladding are alternately laid one on another.

Having the above-mentioned structure, both the core and the cladding ofthe polyimide optical waveguides of the present invention can havealmost the same thermal properties and birefringence.

According to still another of the present invention, there is provided amethod of manufacturing a polyimide optical waveguide, comprising thesteps of: forming a first polyimide layer on a substrate; forming a corelayer having a predetermined refractive index by irradiating thepolyimide layer with an electron beam; forming a second polyimide layeron the core layer and removing the substrate, thereby forming apolyimide film having a two-layer structure; and bonding the secondpolyimide layer as a lower cladding to another substrate, therebyforming a core having a predetermined shape in the core layer.

According to a further aspect of the present invention, there isprovided a method of manufacturing a polyimide optical waveguide,comprising steps of: forming a first polyimide layer on a substrateforming a second polyimide layer on the first polyimide layer, thesecond polyimide layer having a refractive index higher than that of thefirst polyimide layer; forming a third polyimide layer on the secondpolyimide layer, the third polyimide layer having a refractive indexlower than that of the second polyimide layer; and performing electronbeam lithography on the three polyimide layers, thereby forming a corein the second polyimide layer, the core having a predeterminedrefractive index and a predetermined shape.

According to an aspect of the present invention, there is provided amethod of manufacturing a polyimide optical waveguide, comprising thesteps of: forming a plurality of polyimide layers, one on another andirradiating the plurality of polyimide layers with an electron beam,thereby forming cores and claddings alternately laid one on another,each cladding having a refractive index lower than those of the cores.

According to another aspect of the present invention, there is provideda method of manufacturing a polyimide optical waveguide, comprising thesteps of: forming a first polyimide layer on a substrate; and forming acore having a predetermined refractive index and a predetermined size inan upper surface of the polyimide layer and to a predetermined depth, byirradiating the polyimide layer with an electron beam in the conditionthat the election beam is prevented from reaching a lower surface of thepolyimide layer.

Further, with the methods of manufacturing the polyimide opticalwaveguide, according to the present invention, it is possible to formthe core easily, in steps which are easy to perform.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention, and together with the general-description given aboveand the detailed description of the preferred embodiment given below,serve to explain the principles of the invention.

FIG. 1 is a sectional view showing one embodiment of an embedded channelwaveguide in which a core having rectangular section is embedded in onelayer of the cladding of a two-layer structure;

FIG. 2 is a sectional view showing one embodiment of an embedded channelwaveguide in which the core having rectangular section is embedded inthe second layer of the cladding of a three-layer structure;

FIG. 3 is a sectional view showing one embodiment of an embedded channelwaveguide in which the core having semicircle section is embedded in onelayer of the cladding of a two-layer structure;

FIG. 4 is a sectional view showing one embodiment of a channel waveguidein which the core having semicircle section is embedded in the claddingof a one-layer structure;

FIG. 5 is a sectional view showing one embodiment of a channel waveguidein which the core having rectangular section is embedded in one layer ofthe cladding of a two-layer structure;

FIG. 6 is a sectional view showing one embodiment of a ridge channelwaveguide having the structure of forming the core having rectangularsection on the surface of the cladding of a one-layer structure;

FIG. 7 is a sectional view showing one embodiment of a slab waveguidehaving the structure of laminating the core as the second layer on thesurface of the cladding of a one-layer structure

FIG. 8 is a view showing the steps of manufacturing the polyimideoptical waveguide by performing electron beam lithography, which is oneembodiment of the present invention;

FIG. 9 is a view showing the steps of manufacturing an embedded channelpolyimide optical waveguide having a three-layer structure which is oneembodiment of the present invention;

FIG. 10 is a sectional view of an embedded channel polyimide opticalwaveguide of three-layer structure, which is one embodiment of thepresent invention;

FIG. 11 is a view showing the steps of manufacturing an embedded channelwaveguide of a two-layer structure by an electron beam irradiation,which is another embodiment of the present invention;

FIG. 12 is a sectional view of a subject portion of a polyimide opticalwaveguide of a two-layer structure, which is still another embodiment ofthe present invention;

FIG. 13 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 12;

FIG. 14 is a sectional view showing the subject portion of a polyimideoptical waveguide of a three-layer structure, which is anotherembodiment of the present invention;

FIG. 15 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 14;

FIG. 16 is a sectional view showing the subject portion of a polyimideoptical waveguide of a four-layer structure, which is still anotherembodiment of the present invention;

FIG. 17 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 16;

FIG. 18 is a sectional view of the subject portion of showing anotherpolyimide optical waveguide of a four-layer structure;

FIG. 19 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 18;

FIG. 20 is a sectional view of the subject portion of the polyimideoptical waveguide of a five-layer structure, which is another embodimentof the present invention;

FIG. 21 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 20;

FIG. 22 is a sectional view of the subject portion for showing anotherembodiment of the polyimide optical waveguide of a five-layer structure;

FIG. 23 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 22;

FIG. 24 is a sectional view of the subject portion of a polyimideoptical waveguide of a six-layer structure, which is a furtherembodiment of the present invention

FIG. 25 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 24;

FIG. 26 is a sectional view of the subject portion of showing thepolyimide optical waveguide of a six-layer structure;

FIG. 27 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 26;

FIG. 28 is a sectional view of the subject portion of showing anembodiment of a polyimide optical waveguide of a seven-layer structure;and

FIG. 29 is a view showing the steps of manufacturing the polyimideoptical waveguide shown in FIG. 28.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, one embodiment of the present invention will be explainedbased on the drawings.

FIG. 1 shows a sectional view of an embedded channel waveguide of atwo-layer structure, wherein a core 2 is formed on one cladding 1 andanother cladding 3 is laminated on the cladding 1 so as to embed thecore 2. The core 2 used herein is made of polyimide whose refractiveindex is controlled by irradiating with an electron beam. Thepolyimide-includes polyimides formed of a tetracarboxylic acid or aderivative thereof and a diamine, polyimide copolymers, and polyimidemixtures, whose refractive index can be controlled.

A derivative of a tetracarbxylic acid includes an acid anhydride, anacid chloride, or an ester. Examples of a tetracarboxylic acid are(trifluoromethyl)pyromellitic acid, di(trifluoromethyl)pyromelliticacid, di(heptafluoropropyl)pyromellitic acid,pentafluoroethylpyromellitic acid,bis{3,5-di(trifluoromethyl)phenoxy}pyromellitic acid,2,3,3',4'-biphenyltetracarboxylic acid,3,3',4,4'-tetracarboxydiphenylether 2,3',3,4'-tetracaroxydiphenylether,3,3',4,4'-benzophenonetetracaroxylic acid,2,3,6,7-tetracarboxynaphthalene, 1,4,5,7-tetracarboxynaphthalene,1,4,5,6-tetracarboxynaphthalene, 3,3',4,4'-tetracarboxydiphenylmethane,3,3',4,4'-tetracarboxydiphenylsulfone,2,2-bis(3,4-dicarboxyphenyl)propane, 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,5,5'-bis(trifluoromethyl)-3,3',4,4'-teracarboxybiphenyl,2,2',5,5'-tetrakis(trifluoromethyl)-3,3',4,4'-tetracaroxybiphenyl,5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracarboxydiphenylether,5,5'-bis(trifluoromethyl)-3,3',4,4'-tetracaroxybenzophenone, bis{(trifluoromethyl)dicarboxyphenoxy)benzene, bis{(trifluoromethyl)dicarboxyphenoxy}(trifluoromethyl)benzene,bis(dicarboxyphenoxy)(trifluoromethyl)benzene,bis(dicarboxyphenoxy)bis(trifluoromethyl)benzene,bis(dicarboxyphenoxy)tetrakis(trifluoromethyl)benzene,3,4,9,10-tetracarboxyperylene,2,2-bis{(4-(3,4-dicarboxyphenoxy)phenyl}propane, butanetetracarboxylicacid, cyclopentanetetracarboxylic acid, 2,2-bis{4-(3,4-dicarboxyphenoxy)phenyl}hexafluoropropane,bis{(trifluoromethyl)dicarboxyphenoxy}biphenyl,bis{(trifluoromethyl)dicarboxyphenoxy}bis(trifluoromethyl)biphenyl,bis{(trifluoromethyl) dicarboxyphenoxy}diphenylether,bis(dicarboxyphenoxy)bis(trifluoromethyl)biphenyl,bis(3,4-dicarboxyphenyl)dimethylsilane,1,3-bis(3,4-dicarboxyphenyl)tetramethyldisiloxane, difluoropyromelliticacid, 1,4-bis(3,4-dicarboxytrifluorophenoxy)tetrafluorobenzene,1,4-bis(3,4-dicarboxytrifluorophenoxy)octafluorobiphenyl.

Examples of a diamine include m-phenylenediamine, 2,4-diaminotoluene,2,4-diaminoxylene, 2,4-diaminodurene,4-(1H,1H,11H-eicosafluoroundecanoxy)-1,3-diaminobenzene,4-(1H,1H-perfluoro-1-butanoxy)-1,3-diaminobenzene,4-(1H,1H-perfluoro-1-heptanoxy)-1,3-diaminobenzene,4-(1H,1H-perfluoro-1-octanoxy)-1,3-diaminobenzene,4-pentafluorophenoxy-1,3-diaminobenzene,4-(2,3,5,6-tetrafluorophenoxy)-1,3-diaminobenzene,4-(4-fluorophenoxy)-1,3-diaminobenzene,4-(1H,1H,2H,2H-perfluoro-1-hexanoxy)-1,3-diaminobenzene,4-(1H,1H,2H,2H-perfluoro-1-dodecanoxy)-1,3-diaminobenzene,p-phenylenediamine, 2,5-diaminotoluene,2,3,5,6-tetramethyl-p-phenylenediamine, 2,5-diaminobenzotrifluoride,bis(trifluoromethyl)phenylenediamine,diaminotetra(trifluoromethyl)benzene, diamino(pentafluoromthyl)benzene,2,5-diamino (perfluorohexyl)benzene,2,5-diamino(perfluorobuthyl)benzene, benzidine, 2,2'-dimethylbenzidine,3,3'-dimethylbenzidine, 3,3'-dimethoxybenzidine,2,2'-dimethoxybenzidine, 3,3',5,5'-tetramethylbenzidine,3,3'-diacetylbenzidine, 2,2'-bis(trifluoromethyl)-4,4'diaminobiphenyl,octafluorobenzidine, 3,3'-bis(trifluoromethyl)-4,4'-diaminobiphenyl,4,4'-diaminodiphenylether, 4,4'-diaminodiphenylmethane,4,4'-diaminodiphenylsulfone, 2,2-bis(p-aminophenyl)propane,3,3'-dimethyl-4,4'-diaminodiphenylether,3,3'-dimethyl-4,4'-diaminodiphenylmethane, 1,2-bis(anilino)ethane,2,2-bis(p-aminophenyl)hexafluoropropane,1,3-bis(anilino)hexafluoropropane, 1,4-bis(anilino)octafluorobutane,1,5-bis(anilino)decafluoropentane,1,7-bis(anilino)tetradecafluoroheptane,2,2'-bis(trifluoromethyl)-4,4'-diaminodiphenylether,3,3'-bis(trifluoromethyl)-4,4'-diaminodiphenylether,3,3',5,5'-terakis(trifluoromethyl)-4,4'-diaminodiphenylether,3,3'-bis(trifluoromethyl)-4,4'-diaminobenzophenone,4,4"-diamino-p-terphenyl, 1,4-bis(p-aminophenyl)benzene,p-bis(4-amino-2-trifluoromethylphenoxy)benzene,bis(aminophenoxy)bis(trifluoromethyl)benzene, bis(aminophenoxy)tetrakis(trifluoromethyl)benzene, 4,4'"-diamino-p-quarterphenyl,4,4'-bis(p-aminophenoxy)biphenyl,2,2-bis{4-(p-aminophenoxy)phenyl}propane,4,4'-bis(3-aminophenoxy)phenyl)diphenylsulfone,2,2-bis{4-(4-aminophenoxy)phenyl}hexafluoropropane,2,2-bis{4-(3-aminophenoxy)phenyl}hexafluoropropane,2,2-bis{4-(2-aminophenoxy)phenyl}hexafluoropropane,2,2-bis{4-(4-aminophenoxy)-3,5-dimethylphenyl}hexafluoropropane,2,2-bis{4-(4-aminophenoxy)-3,5-ditrifluoromethylphenyl}hexafluoropropane,4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl,4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl,4'-bis(4-amino-2-trifluoromethylphenoxy)diphenylsulfone,4,4'-bis(3-amino-5-trifluoromethylphenoxy)diphenylsulfone,2,2-bis{4-(4-amino-3-trifluoromethylphenoxy)phenyl}hexafluoropropane,bis{(trifluoromethyl)aminophenoxy}biphenyl,bis[{(trifluoromethyl)aminophenoxy}phenyl]hexafluoropropane,diaminoanthraquinone, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene,bis{2-(aminophenoxy)phenyl}hexafluoroisopropylbenzene,bis(2,3,5,6)-tetrafluoro-4-aminophenyl)ether,bis(2,3,5,6)-tetrafluoro-4-aminophenyl)sulfide,1,3-bis(3-aminopropyl)tetramethyldisiloxane,1,4-bis(3-aminopropyldimethylsilyl)benzene,bis(4-aminophenyl)diethylsilane, 1,3-diaminotetrafluorobenzene,1,4-diaminotetrafluorobenzene,4,4'-bis(tetrafluoroaminophenoxy)octafluorobiphenyl.

A fluorinated polyimide obtained from a tetracarboxylic dianhydride anda diamine, either one of, or both of which contain a fluorine atomcombined thereto, is particularly preferably used in the presentinvention. Namely, either one of, or both of a fluorinated dianhydrideand a fluorinated diamine are used to produce a polyimide.

The above mentioned embedded channel waveguide as shown in FIG. 1 can bemanufactured by the following method.

A polyamic acid solution is spin-coated on a substrate such as ofsilicon, and is thermally cured to form a polyimide layer. An electronbeam is irradiated on the entire surface of the polyimide layer tochange the refractive index to a predetermined value. An electronbeam-absorbed dose of the polyimide layer depends on the composition ofthe polyimide, an energy of the electron beam, and the applied amount ofthe electron beam irradiation. The refractive index changes inaccordance therewith approximately.

This polyimide layer irradiated with the electron beam is used as a corelayer.

Next, the same polyamic acid solution is spin-coated on the core layer,and is thermally cured. The material is separated from the substrate toobtain a polyimide film of a two-layer structure.

Then, the polyimide film of a two-layer structure is made to reverse andthe irradiated polyimide layer is made to the upper surface. The lowersurface of the reversed film is adhered by a method such as athermocompression bonding on the other silicon substrate.

Next, the core layer is patterned into a desired shape, for example, arectangular shape, by RIE (reactive ion etching) method.

Finally, the same polyamic acid solution is spin-coated over thepatterned core layer, as an upper cladding layer, and is thermallycured, thereby preparing an embedded channel waveguide.

The step of irradiating the entire surface of the polyimide layer withthe electron beam, as described above, can be applied in forming anembedded channel waveguide, a ridge channel waveguide, and a slabwaveguide, respectively shown in FIGS. 1, 6, and 7. Further, if a metalmask or the like is used, the irradiation step can be applied in formingan embedded channel waveguide and a channel waveguide as shown in FIGS.2, 3, 4, and 5.

In the above-mentioned process, the entire surface of the polyimide filmis irradiated with the electron beam to form the core layer, and the RIEmethod is used at the time of forming the core. Alternatively, theelectron beam irradiation is applied partially on the polyimide layer byusing the metal mask or the like. However, for example, there is anothermethod of forming the core having the predetermined refractive index andsize in which electron beam lithography is applied selectively on thepolyimide layer by means of an electron beam lithography system used inLSI manufacturing, without using the RIE processing. For example, apolyimide solution or a solution of a precursor thereof is applied byspin-coating on a silicon substrate 10, heated to remove the solvent,and cured if necessary, thereby to obtain a polyimide film 11 in step Aas shown in FIG. 8. Next, the electron beam lithography is performed instep B shown in FIG. 8, to form a core 12 having the predeterminedrefractive index and size.

By these two steps B and C shown in FIG. 8, it is possible to obtain anoptical waveguide approximately same as a ridge channel waveguide formedby using the conventional RIE method. That is, it is possible to reducethe number of steps. By using such electron beam lithography method, itis possible to obtain an embedded channel or channel waveguide as shownin FIGS. 2, 3, 4, and 5.

The structure of the optical waveguide manufactured by theabove-mentioned process will be explained with reference to sectionalviews of FIGS. 2 to 7.

FIG. 2 is a sectional view showing one embodiment of the embeddedchannel waveguide in which the core 2 having a rectangular section isembedded in the second layer of the cladding layers 1, 3, and 4 forminga three-layer structure.

FIG. 3 is a sectional view showing one embodiment of the embeddedchannel waveguide in which the core having a semicircle section isembedded in one of the cladding layers forming a two-layer structure.

FIG. 4 is a sectional view showing one embodiment of the channelwaveguide, in which the core having a semicircle section is embedded inthe cladding forming a one-layer structure. In this case, one of thecladding layers forming two-layer structure as shown in FIG. 3 can be ofair.

FIG. 5 is a sectional view showing one embodiment of the channelwaveguide in which the core having a rectangular section is embedded inone of the cladding layers forming a two-layer structure. In this case,the third cladding layer shown in FIG. 2 can be of air.

The structure of the optical waveguide in this invention is not limitedto the embedded optical waveguide and the channel optical waveguide,both mentioned above. It can be applied for a ridge channel waveguidehaving the structure wherein the core having a rectangular section ismounted on the surface of the cladding having a one-layer structure, asshown in the sectional view of FIG. 6. Further, as shown in thesectional view of FIG. 7, it can be applied for a slab waveguide havingthe structure wherein the core used as the second layer is laminated onthe surface of the cladding having a one-layer structure.

Furthermore, the manufacturing of a waveguide with a guiding conditionsuch as single-mode or a multi-mode are realized by controlling acore-cladding refractive index ratio, using electron beam irradiation.

Further, even if the above-mentioned electron beam lithography system isnot used, it is possible to form the core layer by irradiating thepolyimide with the electron beam through an ordinary photomask.

Furthermore, as the means for forming an embedded channel waveguide, thecore layer film can be sandwiched by polyimide films which will becomecladdings, and these components can be compressed or adhered throughthin adhesive agent layers.

Hereinafter, the reference will be made to the specific Examples of thepresent invention. However, the present invention is not restricted tothese Examples.

EXAMPLE 1

88.8 g (0.2 mol) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride, 64.0 g (0.2 mol) of2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and 1000 g ofN,N-dimethylacetamide were added in a conical flask. The mixture wasstirred for three days at the room temperature under nitrogen atmosphereto obtain a polyamic acid solution having a concentration of about 15%by weight. The polyamic acid solution was spin-coated on a siliconwafer, and heated at 70° C. for two hours, at 160° C. for one hour, at250° C. for 30 minutes, and at 380° C. for one hour in an oven toimidize it, thereby obtaining a polyimide layer with 10 μm thickness. Asa result of measuring the refractive index of the film at wavelength of1.3 μm, the refractive index in the direction parallel to the filmsurface (TE) was 1.521 and the refractive index in the directionperpendicular to the film surface (TM) was 1.513. Further, a thermalexpansion coefficient was 8.2×10⁻⁵ and a glass transition temperaturewas 335° C.

The polyimide layer formed on the silicon substrate was irradiated withthe electron beam having an energy of 400 Kev for about 30 minutes atroom temperature at a dose of 5×10¹⁵ e/cm². The measured refractiveindices of this polyimide layer were 1.526 for TE direction and 1.517for TM direction respectively. Further, the thermal expansioncoefficient was 8×10⁻⁵, and the glass transition temperature was 330° C.

After the same polyamic acid solution was spin-coated on theelectron-beam irradiated polyimide, the solution was heated at 70° C.for two hours, at 160° C. for one hour, at 250° C. for 30 minutes, andat 380° C. for one hour in the oven for imidization, thereby obtaining apolyimide film having a thickness of 30 μm and a two-layer structure.This polyimide film was separated from the silicon substrate, theirradiated polyimide surface was made to be an upper layer, and thelower layer was adhered to another silicon substrate.

Next, using an electron beam vapor deposition system, a 0.3 μm thickaluminum film was formed on the polyimide film. After an usual positiveresist was applied by the spin-coating method, prebake was performed.Then, ultraviolet rays were applied through a mask for forming a patternhaving a line width of 10 μm and a length of 60 mm by using anultra-high pressure mercury lamp, and the resist was developed by usinga developer for positive resist. Thereafter, afterbake was performed.Next, the aluminum film was wet etched at each portion not coated withthe resist. After washing and drying, the RIE processing of thepolyimide was performed by using a dry etching system. The aluminumremaining on the polyimide was removed by applying the above-mentionedetching solution to obtain a ridge channel waveguide having a core widthof 10 μm .

Further, the same polyamic acid solution was spin-coated on the ridgechannel waveguide, and was heated at 70° C. for two hours, at 160° C.for one hour, at 250° C. for 30 minutes, and at 380° C. for one hour inthe oven to achieve imidization. Thus, an upper cladding layer wasformed. In this manner, an embedded channel waveguide was obtained inwhich the core-cladding refractive index ratio was almost same at TE andTM (0.005 for TE, 0.004 for TM). Further, the core and the cladding ofthis waveguide had almost the same thermal properties.

As mentioned above, it is possible to manufacture a polyimide opticalwaveguide whose core and cladding have almost the same thermalproperties and birefringence.

Next, another method of manufacturing a polyimide optical waveguide,using the polyimide multi-layer film of the present invention, will bedescribed with reference to another embodiment.

FIG. 9 is a view showing the steps in forming an embedded channelwaveguide having a three-layer structure, according to the presentinvention. FIG. 10 is a sectional view of the embedded channelwaveguide. According to the method of the present invention, it ispossible to form the waveguide in four steps. Unlike in the conventionalmethod using the RIE method, it is possible to reduce the number ofsteps. Namely, a polyimide solution or a precursor solution is coatedby, for example, spin-coating method on the substrate 10 of silicon orthe like, in the step A shown in FIG. 9. The solution is heated toremove the solvent, and cured, if necessary, thereby to obtain a firstpolyimide layer as shown in step B shown in FIG. 9. Then, a secondpolyimide layer 13 is formed on the first polyimide layer 11, in step Cshown in FIG. 9 by using a polyimide material whose refractive index islarger than that of the first layer. Further, a third polyimide layer 14is formed on the second polyimide layer 13 in step D shown in FIG. 9 byusing a polyimide material whose refractive index is smaller than thatof the second layer 13. The desired pattern is drawn on the polyimidefilm having the three-layer structure with an electron beam, so as tohave a predetermined refractive index and a predetermined size. Therelationship of refractive indices of these polyimide layers is n1<n2,and n3<n2, where the refractive index of the first polyimide layer isn1, that of the second polyimide layer is n2, and that of the thirdpolyimide layer is n3. If changes of the refractive indices of the threepolyimide layers are represented by Δn1, Δn2, and Δn3, respectively, bythe electron beam irradiation, the refractive index of a region 1 ofFIG. 10 is n3, that of a region 2 is n3+Δn3, that of a region 3 is n3,that of a region 4 is n2, that of a region 5 is n1, that of a region 6is n1+Δn1, that of a region 7 is n1, that of a region 8 is n2, and thatof a region 9 is n2 +Δn2. The materials and the irradiation conditionare selected such that each difference (Δn1-Δn2, Δn2-Δn3, Δn3-Δn1) ofthe refractive index change Δn1, Δn2, Δn3 caused by the electron beamirradiation is less than the refractive index difference between theoriginal materials. Hence, the region 9 having the largest refractiveindex becomes the core in which the light is confined and guided.

When the electron beam is applied onto the polyimide film, measures aretaken, if necessary, to prevent deflection of the electron beam causedby a charge of a static electricity. To this end, for example, aconductive film may be deposited on the polyimide film, or a conductivemesh may be compressed on the polyimide film.

Furthermore, it is possible to perform various pretreatments for thematerial to be irradiated with the electron beam. For example, thematerial can be left in the atmosphere which is filled with a substanceand be irradiated with an electron beam to perform the electron beamirradiation effectively. It is possible to set the pretreatmentconditions freely in accordance with the objective.

After the core is formed by the electron beam lithography, if the coreis sandwiched, at its upper and lower sides, between by the polyimidelayers having lower refractive indices than that of the core, it ispossible to obtain an embedded channel waveguide. The core is sandwichedthrough adhesive layers, by using thermocompression bonding, or by usingspin-coating.

The optical waveguide may have various shapes; it can be a linear,curved, folded, S-shpaed, tapered, branched, or crossing opticaldirectional coupler. It can be a two-mode waveguide coupler, or agrating. Further, the width of the core can be set freely.

Hereinafter, the present invention will be described in detail, withreference to Examples. However, the present invention is not limited tothese Examples. Typical Examples will be shown below though it ispossible to manufacture the various kinds of polyimide optical waveguideby combining materials and shapes in various ways.

EXAMPLE 2

88.8 g (0.2 mol) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride, 64.0 g (0.2 mol) of2,2-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and 1000 g ofN,N-dimethylacetamide were added in a conical flask. The mixture wasstirred for three days at room temperature under nitrogen atmosphere toobtain a polyamic acid solution having a concentration of about 15% byweight. After the polyamic acid solution was spin-coated on a siliconwafer, it was heated at 70° C. for two hours, at 160° C. for one hour,at 250° C. for 30 minutes, and at 350° C. for one hour in an oven toachieve imidization, thereby forming a polyimide layer with 10 μmthickness. Then, aluminum was deposited on the polyimide layer to athickness of 10 nm within a vacuum deposition system, and the resultantstructure was introduced into an electron beam lithography system.

Further, a pattern having a width of 8 μm and a length of 66 mm wasdrawn on the polyimide layer by using an electron beam having energy of25 Kev at a dose of 1500 μC/cm².

Thereafter, the aluminum film was removed by using an etching solutionto obtain a polyimide optical waveguide. End of the obtained polyimideoptical waveguide was optically polished, and a light having awavelength of 633 nm was introduced thereinto. It was confirmed that thelight was confined in the core and was guided. Further a light having awavelength of 1.3 μm was introduced within the core of the waveguide viaa single mode optical fiber, and the output light was received by amulti-mode optical fiber. When the intensity of the output light wasmeasured, it was confirmed that the loss was 1 dB/cm or less, includingthe coupling loss.

EXAMPLE 3

Aluminum was deposited on the polyimide film formed in the same way asin Example 2, and was introduced into the electron beam lithographysystem.

A pattern having a width of 10 μm and a length of 66 mm was drawn on thefilm by using an electron beam having energy of 25 Kev at a dose of 900μC/cm².

Thereafter, the loss of the light having 1.3 μm of wavelength wasmeasured for the waveguide prepared in this Example. It was 1 dB/cm orless, including the coupling loss.

EXAMPLE 4

88.8 g (0.2 mol) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride 64.0 g (0.2 mol) of2,2-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and 1000 g ofN,N-dimethylacetamide was added in a conical flask. The mixture wasstirred for three days at room temperature under nitrogen atmosphere toobtain a polyamic acid solution having a concentration of about 15% byweight (hereinafter referred to as Solution A). Solution A wasspin-coated on an optically polished aluminum plate and heated at 70° C.for two hours, at 160° C. for one hour, at 250° C. for 30 minutes, andat 350° C. for one hour in the oven to achieve imidization, therebyforming a polyimide layer having a thickness of 10 μm. The meanrefractive index of this layer relative to light having a wavelength of1.3 μm was 1.519.

Instead of 0.2 mol of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride, 0.14 mol of 2,2-bis (3,4-dicarboxyphenyl)hexafluoropropanedianhydride and 0.06 mol of pyromellitic acid dianhydride were used toprepare a polyamic acid solution (hereinafter referred to as Solution B)and a polyimide layer in the manner described above. The refractiveindex of this layer exhibited to the light having 1.3 μm wavelength was1.527.

Solution A was spin-coated on an optically polished aluminum substrate,such that a cured layer has a thickness of 20 μm. The solution washeated at 70° C. for two hours, at 160° C. for one hour, at 250° C. for30 minute, and at 380° C. for one hour in the oven to obtain a firstpolyimide layer. Further, Solution B was spin-coated thereon such that acured layer has a thickness of 8 μm, and a second polyimide layer wasprepared under the same curing condition. Aluminum was deposited to athickness of 10 nm on the film having the two-layer structure in thevacuum deposition system, and the resultant structure was introducedinto the electron beam lithography system. The polyimide film wasirradiated with an electron beam having energy of 25 keV, over a widthof 8 μm and a length of 66 mm, at a dose of 1500 μC/cm². Then, thealuminum film was removed by an etching solution, thereby obtaining apolyimide optical waveguide.

The end of the obtained polyimide optical waveguide were opticallypolished, and a light having a wavelength of 633 nm was introducedthereinto. It was confirmed that the light was confined in the secondlayer and was guided. Further, a light having a wavelength of 1.3 μm wasfound to be confined in the region (8 μm×8 μm) and be guided to theoutput end of the waveguide.

EXAMPLE 5

Solution A was spin-coated on the film having the two-layer structureprepared in the same way as in Example 4, and thermally cured under thesame curing conditions to form a third polyimide layer having athickness of 20 μm.

On the thus obtained polyimide film having the three-layer structure,the electron beam lithography was performed in the same manner as inExample 4, by using the electron beam lithography system.

The end of the obtained polyimide optical waveguide was opticallypolished, and a light having a wavelength of 633 nm was introducedthereinto. It was confirmed that the light was confined in the secondlayer and was guided. Further, as for the light having a wavelength of1.3 μm, it was confirmed that the light was confined in the region of 8μm×8 μm, and was guided to the another end of the waveguide.

EXAMPLE 6

On the film having a three-layer structure prepared in the same way asin Example 5, Solution B was spin-coated so as to obtain a thickness of8 μm after the heat curing, and cured under the same curing condition toform a fourth layer. Further, Solution A was spin-coated thereon so asto obtain a thickness of 20 μm after the heat curing, and cured underthe same curing condition to form a fifth layer.

Electron beam lithography was performed on the polyimide film having thefive-layer structure in the same condition as in Example 4, by using theelectron beam lithography system.

The end of the obtained polyimide optical waveguide having themulti-layer structure was optically polished, and a light having awavelength of 633 nm was introduced thereinto. It was confirmed that thelight was confined in the second and fourth layers. Further, as for thelight having a wavelength of 1.3 μm, it was confirmed that the light wasconfined in the regions of 8 μm×8 μm of the second and fourth layers andwas guided to the another end of the waveguide.

EXAMPLE 7

Solution A was spin-coated on a film having a two-layer structureprepared in the same way as in Example 4, so as to obtain a thickness of20 μm after the heat curing, and cured under the same curing conditionto form a third layer. Aluminum was deposited to a thickness of 10 nm onthe film having the three-layer structure within the vacuum depositionsystem. The resultant structure was introduced into the electron beamlithography system. Then, this film was irradiated with an electron beamhaving energy of 50 keV, over a region of 8 μm (width)×66 mm (length) ata dose of 1500 μC/cm². Thereafter, the aluminum film was removed byusing an etching solution to obtain a polyimide optical waveguide.

The end of the obtained polyimide optical waveguide was polishedoptically, and a light having a wavelength of 633 nm was introducedthereinto. It was confirmed that the light was confined in the secondlayer and was guided. Further, as for the light having a wavelength of1.3 μm, it was confirmed that the light was confined in the region of 8μm×8 μm, and was guided to the another end of the waveguide.

EXAMPLE 8

On a film having a three-layer structure prepared in the same way as inExample 5, Solution B was spin-coated so as to obtain a thickness of 8μm after the heat curing, and cured under the same curing condition toform a fourth layer. Further, Solution A was spin-coated thereon so asto obtain a thickness of 10 μm after the heat curing, and cured underthe same curing condition to form a fifth layer.

Electron beam lithography was performed on the polyimide film having thefive-layer structure, in the same way as in Example 7, by using theelectron beam lithography system.

An end of the obtained polyimide optical waveguide having themulti-layer structure was polished optically, and light having awavelength of 633 nm was applied to the end. It was confirmed that thelight was confined in the second and fourth layers and was guided. Asfor the light having a wavelength of 1.3 μm, it was confirmed that thelight was confined in the regions of 8 μm×8 μm of the second and fourthlayers and was guided to the another end of the waveguide.

Next, a method of preparing the polyimide optical waveguide according toanother embodiment of the present invention will be described withreference to FIG. 11.

After a polyimide solution or a polyamic acid solution was applied byspin-coating on a substrate 10 as shown in FIG. 11, the resultantstructure was heated, the solvent was removed, and the structure wascured, if necessary, thereby to obtain a polyimide film 11 in step Ashown in FIG. 11. Then, an electron beam energy was selected, and theelectron beam was applied to the film 11, thus changing the refractiveindex of the film 11 to a predetermined value and forming thepredetermined size thereof, to form a core 12 having the predetermineddepth in step B shown in FIG. 11. The electron beam must be applied insuch a condition as not to permeate to the rear surface of the polyimidefilm 11. In this manner, a channel waveguide can be prepared.

Further, it is possible to form an upper cladding 14 in step C shown inFIG. 11 by means of spin-coating a polyimide solution or a polyamic acidsolution on the lower cladding.

With the manufacturing method described above, it is possible to omitthe step of bonding the polyimide films and to omit the step ofperforming spin-coating on the polyimide film separated from thesubstrate. This further simplifies the manufacturing method.

In the steps A to C shown in FIG. 11, the polyimide material can betreated in Various ways before electron beam irradiation. For example,the electron beam irradiation can be achieved effectively by holding thematerial in an atmosphere filled with a substance. It is possible toperform pretreatment freely, in accordance with the objective.

As for the shape of the optical waveguide, the path can be a liner,curved, folded, S-shaped, tapered, branched or crossing opticaldirection coupler. Furthermore, it can be a two-mode waveguide coupler,or a grating. Further, the core can have any width and depth desired.

Specific examples will be explained. Although it is possible tomanufacture the various kinds of polyimide optical waveguides bychanging the combination of the material and the shape, typecialexamples are as follows.

EXAMPLE 9

88.8 g (0.2 mol) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride, 64.0 g (0.2 mol) of2,2-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and 1000 g ofN,N-dimethylacetamide were added in a conical flask. The mixture wasstirred for three days at room temperature under nitrogen atmosphere toobtain polyamic acid having a concentration of about 15% by weight. Thepolyamic acid solution was spin-coated on a silicon wafer, and heated at70° C. for two hours, at 160° C. for one hour, at 250° C. for 30minutes, and at 350° C. for one hour in an oven, to achieve imidization.Thus, a polyimide film having a thickness of 50 μm was obtained. Thisfilm was introduced into an electron beam lithography system.

Then, this film was irradiated with an electron beam having energy of 10kev at a dose of 1500 μC/cm², thus forming a pattern having a width of 8μm and a length of 60 mm.

An end of the polyimide optical waveguide, thus formed was polishedoptically, and a light having a wavelength of 1.3 μm was introducedthereinto. When the other end of the waveguide was observed by using amicroscope with an infrared camera, it was confirmed that the light wasconfined in the waveguide.

EXAMPLE 10

88.8 g (0.2 mol) of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanedianhydride, 64.0 g (0.2 mol) of2,2-bis(trifluoromethyl)-4,4'-diaminobiphenyl, and 1000 g ofN,N-dimethylacetamide were added in a conical flask. The mixture wasstirred for three days at room temperature under nitrogen atmosphere, toobtain a polyamic acid solution having a concentration of about 15% byweight. This polyamic acid solution was spin-coated on a silicon wafer,and heated at 70° C. for two hour, at 160° C. for one hour, at 250° C.for 30 minutes, and at 350° C. for one hour in the oven to achieveimidization. Thus, a polyimide layer having a thickness of 50 μm wasobtained. This layer was introduced into an electron beam lithographysystem.

Then, an electron beam having an energy of 10 kev was applied onto thisfilm at a dose of 1500 C/cm², thereby forming a pattern having a widthof 8 μm a length of 60 mm.

Thereafter, the polyamic acid solution was spin-coated on the polyimidelayer formed above and cured to obtain an embedded channel polyimideoptical waveguide. The end of the polyimide optical waveguide waspolished optically, and light having a wavelength of 1.3 μm wasintroduced thereinto. When the another end of the waveguide was observedusing a microscope with an infrared camera, it was confirmed that thelight was confined in the core of the waveguide.

This embedded channel polyimide optical waveguide could be made in onlythree steps, in less steps than in the conventional method. Further, itcould be manufactured without bonding between polyimide films andwithout performing spin-coating on the separated polyimide film.

Embodiments of the polyimide optical waveguides having two or morepolyimide layers, and a method of manufacturing waveguides, whereincores and cladding are formed alternately in the predetermined regionsof the layers will be explained with reference to FIGS. 12 to 29.

FIG. 12 is a sectional view of the subject portion of a polyimideoptical waveguide having a two-layer structure. The optical waveguidehas a polyimide layer 20 having a predetermined refractive index. Asecond polyimide layer 21 having a refractive index higher than that ofthe polyimide layer 20 is formed on the layer 20. A cladding 23 isformed in the polyimide layer 20 by applying an electron beam from theupper side to the predetermined region, and a core 24 is formed in thepolyimide layer 21.

The process of manufacturing the core and the cladding of the waveguideshown in FIG. 12 will be described with reference to FIG. 13.

The polyimide layer 20 is formed on the substrate in step A shown inFIG. 13, and the polyimide layer 21 is formed on the layer 20 in step Bshown in FIG. 13. Then, an electron beam is directed from above theupper side of the polyimide layer 21 and hence to the substrate 10. Thecladding 23 is thereby formed in the polyimide layer 20, and the core 24is thus formed in the second polyimide layer 21 over the regionirradiated with the electron beam, in step C shown in FIG. 13.

FIG. 14 is a sectional view of the subject portion of a polyimideoptical waveguide having a three-layer structure. The waveguide has apolyimide layer 21 having a predetermined refractive index. A secondpolyimide layer 20 having a refractive index lower than that of thepolyimide layer 21 is formed on the layer 21. Further a third polyimidelayer 22 having a refractive index higher than that of the secondpolyimide layer 20 is formed on the second polyimide layer 20. A core 24is formed in the polyimide layer 21 by applying an electron beam ontothe upper side and hence to the substrate 10. A cladding 23 is formed inthe polyimide layer 20, and another core 25 is formed in the polyimidelayer 22.

The process of manufacturing the core and the cladding of the waveguideshown in FIG. 14 will be described with reference to FIG. 15.

The polyimide layer 21 is formed on the substrate in step A shown inFIG. 15. The second polyimide layer 20 is formed on the polyimide layer21 in step B shown in FIG. 15. Further, in step C shown in FIG. 15, thethird polyimide layer 22 is formed on the second polyimide layer 20.

Then, in step D shown in FIG. 15, an electron beam is directed fromabove the upper side of the polyimide layer 22 and hence to thesubstrate 10. The core 24 is thereby formed in the polyimide layer 20,the cladding 23 is thereby formed in the second polyimide layer 21, andthe core 25 is formed in the third polyimide layer 22 over the regionirradiated with the electron beam in step D shown in FIG. 15.

FIG. 16 is a sectional view of the subject portion of a polyimideoptical waveguide having a four-layer structure. A second polyimidelayer 20 having a refractive index lower than that of the polyimidelayer 21 is formed on a first polyimide layer 21 having a predeterminedrefractive index. A third polyimide layer 22 having a refractive indexhigher than that of the second polyimide layer 20 is formed on thesecond polyimide layer 20, and the fourth polyimide layer 26 having arefractive index lower than that of the polyimide layer 22 is formed onthe third polyimide layer 22. In a prescribed region, a core 24 formedby irradiating the upper side with an electron beam is located in thepolyimide layer 21, a cladding 23 is formed in the polyimide layer 20, acore 25 is formed in the polyimide layer 22, and a cladding 27 is formedin the polyimide layer 26.

The process of manufacturing the cores and the claddings of thewaveguide shown in FIG. 16 will be explained with reference to FIG. 17.

The polyimide layer 21 is formed on the substrate in step A shown inFIG. 17. The second polyimide layer 20 is formed on the polyimide layer21 in step B shown in FIG. 17. Further, in step C shown in FIG. 17, thethird polyimide layer 22 is formed on the second polyimide layer 20.Then, in step D shown in FIG. 17, the fourth polyimide layer 26 isformed on the third polyimide layer 22.

In step E shown in FIG. 17, an electron beam is applied to the upperside of the polyimide layer 26 and hence to the substrate 10. As aresult, the core 24 is formed in the polyimide layer 21, the cladding 23is formed in the second polyimide layer 20, the core 25 is formed in thethird polyimide layer 22, and the cladding 27 is formed in the fourthpolyimide layer 26, over the region irradiated with the electron beam.

Next, another embodiment of a polyimide optical waveguide having afour-layer structure will be described with reference to the sectionalview of FIG. 18.

On a polyimide layer 20 having a predetermined refractive index, asecond polyimide layer 21 having a refractive index higher than that ofthe polyimide layer 20 is formed. A third polyimide layer 26 having arefractive index lower than that of the second polyimide layer 21 isformed on the second polyimide layer 21, and the fourth polyimide layer22 having a refractive index higher than that of the polyimide layer 26is formed on the third polyimide layer 26. A cladding 23 is formed inthe polyimide layer 21 by irradiating an electron beam to the upper sideand hence to a predetermined region. A cladding 27 is formed in thepolyimide layer 26, and the core 25 is formed in the polyimide layer 22.

The step of manufacturing the cores and the claddings of the waveguideshown in FIG. 18 will be explained with reference to FIG. 19.

The polyimide layer 20 is formed on the substrate 10, in step A shown inFIG. 19. The second polyimide layer 21 is formed on the polyimide layer20 in step B shown in FIG. 19. Further, in step C shown in FIG. 19, thethird polyimide layer 26 is formed on the second polyimide layer 21. Asshown in step D shown in FIG. 19, the fourth polyimide layer 22 isformed on the third polyimide layer 26.

Then in step E shown in FIG. 19, an electron beam is applied to theupper side of the polyimide layer 22 and hence to the substrate 10. As aresult, the cladding 23 is formed in the polyimide layer 20, the core 24is formed in the second polyimide layer 21, the cladding 27 is formed inthe third polyimide layer 26, and the core 25 is formed in the fourthpolyimide layer 22, over the region irradiated with the electron beam.

FIG. 20 is a sectional view of the subject portion of a polyimideoptical waveguide having a five-layer structure, wherein a fifthpolyimide layer 28 is formed on the fourth layer 26 of the waveguide ofFIG. 16. The fifth layer 28 has a refractive index higher than that ofthe fourth layer 26. A core 24 is formed in the polyimide layer 21 byapplying an electron beam to the upper side of the fifth polyimide layer28. A cladding 23 is formed in the polyimide layer 20, a core 25 isformed in the polyimide layer 22, a cladding 27 is formed in thepolyimide layer 26, and a core 29 is formed in the fifth polyimide layer

The steps of manufacturing the cores and the claddings of the polyimideoptical waveguide shown in FIG. 20 will be explained with reference toFIG. 21.

The polyimide layer 21 is formed on the substrate in step A shown inFIG. 21. The second polyimide layer 20 is formed on the polyimide layer21 in step B shown in FIG. 21. Further, in step C shown in FIG. 21, thethird polyimide layer 22 is formed on the second polyimide layer 20. Instep D shown in FIG. 21, the fourth polyimide layer 26 is formed on thethird polyimide layer 22. Then, in step E shown in FIG. 21, thepolyimide layer 28 is formed on the polyimide layer 26.

In step F shown in FIG. 21, an electron beam is applied to the upperside of the fifth polyimide layer 28 and hence to the substrate 10. As aresult, the core 24 is formed in the polyimide layer 21, the cladding 23is in the second polyimide layer 20, the core 25 is in the thirdpolyimide layer 22, the cladding 27 is in the fourth polyimide layer 26,and the core 29 is in the fifth polyimide layer 28.

A further embodiment of a polyimide optical waveguide having afive-layer structure will be described with reference to the sectionalview of FIG. 22.

The first to fourth layers of this embodiment are formed in the same wayas the layers of the polyimide optical waveguide (FIG. 19) having afour-layer structure, which are formed in steps A to D shown in FIG. 19.

A step E shown in FIG. 23 is the step of forming the fifth polyimidelayer 30 having a refractive index lower than that of the polyimidelayer 22, on the fourth polyimide layer 22.

An electron beam is applied to the upper side of the fifth polyimidelayer 30 and hence to the substrate 10 in step F shown in FIG. 23, acladding 23 is formed in the polyimide layer 20, a core 24 is in thesecond polyimide layer 21, a cladding 27 is in the third polyimide layer26, a core layer 25 is in the fourth polyimide layer 22, and a cladding31 is in the fifth polyimide layer 30, over the region irradiated withthe electron beam.

FIG. 24 is a sectional view of the subject portion of a polyimideoptical waveguide having a six-layer structure, wherein a sixthpolyimide layer 30 is formed on the fifth polyimide layer 28 of thewaveguide of FIG. 20. The sixth layer 30 has a refractive index lowerthan that of the fifth layer. A core 24 is formed in the polyimide layer21 by applying an electron beam to the upper side of the sixth polyimidelayer 30. A cladding 23 is formed in the polyimide layer 20, a core 25is in the polyimide layer 22, a cladding 27 is in the polyimide layer26, a core 29 is in the fifth polyimide layer 28, and a cladding 31 isin the sixth polyimide layer 30.

The step of manufacturing the cores and the claddings of the waveguideshown in FIG. 24 will be explained with reference to FIG. 25.

The first to fifth layers of this embodiment are formed in the same wayas the layers of the polyimide optical waveguide (FIG. 21) having afive-layer structure are formed in steps A to E shown in FIG. 21.

The step F shown in FIG. 25 is the step of forming the sixth polyimidelayer 30 having a refractive index lower than that of the polyimidelayer 28, on the fifth polyimide layer 28.

An electron beam is applied to the upper side of the sixth polyimidelayer 30 and hence to the substrate 10, in step G shown in FIG. 25. As aresult, the core 24 is formed in the polyimide layer 21, the cladding 23is in the second polyimide layer 20, the core 25 is in the thirdpolyimide layer 22, the cladding 27 is in the fourth polyimide layer 26,the core 29 is in the fifth polyimide layer 28, and the cladding 31 isin the sixth polyimide layer 30, over the region irradiated with theelectron beam.

Next, another embodiment of a polyimide optical waveguide having asix-layer structure will be described with reference to the sectionalview of FIG. 26.

The first to fifth layers of this embodiment are formed in steps A to Eshown in FIG. 27 in the same way as the fist to fifth layers of thepolyimide optical waveguide (FIG. 22) having a five-layer structure areformed in steps A to E shown in FIG. 23.

The step F shown in FIG. 27 shows the step of forming the sixthpolyimide layer 28 having a refractive index higher than that of thepolyimide layer 30, on the fifth polyimide layer 30.

An electron beam is applied to the upper side of the sixth polyimidelayer 28 and hence to the substrate 10 in step G shown in FIG. 27. As aresult, the cladding 23 is formed in the polyimide layer 20, the core 24is in the second polyimide layer 21, the cladding 27 is in the thirdpolyimide layer 26, the core 25 is in the fourth polyimide layer 22, thecladding 31 is in the fifth polyimide layer 30, and the core 29 is inthe sixth polyimide layer 28, over the region irradiated with theelectron beam.

FIG. 28 is a sectional view of the subject portion of a polyimideoptical waveguide having a seven-layer structure, wherein a seventhpolyimide layer 32 is formed on the sixth layer 28 of the waveguide ofFIG. 26. The seventh layer 28 has a refractive index lower than that ofthe sixth layer 28. The cladding 23 is formed in the polyimide layer 20by irradiating the upper side of the seventh polyimide layer 32 with anelectron beam. The core 24 is formed in the polyimide layer 21, thecladding 27 is in the polyimide layer 26, the core 25 is in thepolyimide layer 22, the cladding 31 is in the fifth polyimide layer 30,the core 29 is in the sixth polyimide layer 28, and the cladding 33 isin the seventh polyimide layer 32.

The step of manufacturing the cores and claddings of the waveguide shownin FIG. 28 will be described with reference to FIG. 29.

The first to sixth layers of this embodiment are formed in steps A to Fshown in FIG. 27 in the same way as the first to sixth layers of thepolyimide optical waveguide (FIG. 26) having a six-layer structure.

The step G shown in FIG. 29 shows the step of forming the seventhpolyimide layer 32 having a refractive index lower than that of thesixth layer, on the sixth polyimide layer 28.

An electron beam is applied to the upper side of the seventh polyimidelayer 32 and hence to the substrate 10, in step H shown in FIG. 29. As aresult, the cladding 23 is formed in the polyimide layer 20, the core 24is in the second polyimide layer 21, the cladding 27 is in the thirdpolyimide layer 26, the core 25 is in the fourth polyimide layer 22, thecladding 31 is in the fifth polyimide layer 30, the core 29 is in thesixth polyimide layer 28, and the cladding 33 is in the seventhpolyamide layer 32.

As mentioned above, the present invention provides a improved polyimideoptical waveguide having two or more polyimide layers, and also a methodof manufacturing the waveguide in which any core and any cladding areformed alternately in a predetermined region of each layer.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and illustrated examples shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method of manufacturing a polyimide opticalwaveguide, comprising the steps of:forming a core made of a polyimidewhose refractive index is controlled to be a predetermined value byelectron beam irradiation; and forming a cladding set in contact withsaid core, and said cladding being made of the same material as saidcore and having a refractive index lower than that of said core.
 2. Amethod of manufacturing a polyimide optical waveguide according to claim1, wherein said polyimide is a fluorinated polyimide.
 3. A method ofmanufacturing a polyimide optical waveguide, comprising the stepsof:forming a first polyimide layer on a substrate; forming a core layerhaving a predetermined refractive index by irradiating said polyimidelayer with an electron beam; forming a second polyimide layer on saidcore layer and removing said substrate, thereby forming a polyimide filmhaving a two-layer structure; and bonding said second polyimide layer asa lower cladding to another substrate, thereby forming a core having apredetermined shape in said core layer.
 4. The method according to claim3, wherein said polyimide is fluorinated polyimide.
 5. The methodaccording to claim 3, wherein said step of forming said core layercomprises performing an RIE method on said core layer.
 6. The methodaccording to claim 3, wherein said step of bonding said second layer tothe other substrate includes a (subsequent) step of spin-coating apolyamic acid solution on said second polyimide layer containing said,thereby forming an upper cladding by heat curing said polyamic acid. 7.A method of manufacturing a polyimide optical waveguide, comprising thesteps of:forming a first polyimide layer on a substrate; forming asecond polyimide layer on said first polyimide layer, said secondpolyimide layer having a refractive index higher than that of said firstpolyimide layer; forming a third polyimide layer on said secondpolyimide layer, said third polyimide layer having a refractive indexlower than that of said second polyimide layer; and performing electronbeam lithography on said three polyimide layers, thereby forming a corein said second polyimide layer, said core having a predeterminedrefractive index and a predetermined shape.
 8. The method according toclaim 7, wherein said first, second and third polyimide films are madeof fluorinated polyimide.
 9. A method of manufacturing a polyimideoptical waveguide, comprising the steps of:forming a plurality ofpolyimide layers, one on another; and irradiating said plurality ofpolyimide layers with an electron beam, thereby forming cores andcladdings alternately laid one on another, each cladding having arefractive index lower than those of said cores.
 10. The methodaccording to claim 9, wherein said step of forming a plurality ofpolyimide layers comprises forming two polyimide layers, one on theother, said two polyimide layers having different refractive indices.11. The method according to claim 9, wherein said step of forming aplurality of polyimide layers comprises forming seven polyimide layers,one on another, said seven polyimide layers having different refractiveindices.
 12. A method of manufacturing a polyimide optical waveguide,comprising the steps of:forming a first polyimide layer on a substrate;and forming a core having a predetermined refractive index and apredetermined size in an upper surface of said polyimide layer and to apredetermined depth, by irradiating said polyimide layer with anelectron beam in the condition that the election beam is prevented fromreaching a lower surface of said polyimide layer.
 13. The methodaccording to claim 12, further comprising a step of forming an uppercladding on said polyimide layer and also on said core.
 14. The methodaccording to claim 12, wherein said polyimide layer is made offluorinated polyimide.